This invention relates generally to citrus waste processing, and more particularly to pretreatment of citrus waste, primarily peel, to remove antimicrobial substances, to pasteurize or sterilize such waste and aid subsequent saccharification and fermentation to ethanol or other products. More specifically, the present invention relates to a method of pretreating citrus waste to remove peel oil components (e.g., limonene) involving preheating the citrus waste through indirect heating in a preheater reactor to form preheated citrus waste and conveying the preheated citrus waste to a main reactor, heating the preheated citrus waste through a combination of (simultaneous) indirect heating and direct heating in a main reactor to produce pasteurized citrus waste solids and a vapor containing water and peel oil components, and separating the pasteurized citrus waste solids and the vapor containing water and peel oil components to produce separated pasteurized citrus waste solids low in peel oil content and with disrupted cell structure and a separated vapor containing water and peel oil components. The method optionally further involves cooling the separated pasteurized citrus waste solids and saccharifying and fermenting the cooled pasteurized citrus waste solids using enzymes and yeasts or other microorganisms. The method optionally further involves reducing the particle size of the citrus waste prior to preheating.
Additionally, the present invention relates to a system for pretreating citrus waste to remove peel oil components from the citrus waste involving conveying means to convey citrus waste into and through the system; indirect preheating means to preheat the citrus waste; indirect and direct heating means to heat the preheated citrus waste and produce pasteurized citrus waste solids and a vapor containing water and peel oil components; separating means to separate the pasteurized citrus waste and vapor containing water and peel oil components to produce separated pasteurized citrus waste solids and separated vapor containing water and peel oil components; heat recovery means to convey separated vapor containing water and peel oil components to the indirect preheating means; and optionally size reducing means to reduce the particle size of the citrus waste prior to preheating. The system optionally includes cooling means for cooling the separated pasteurized citrus waste solids and means for saccharifying and fermenting such cooled waste.
Development of industrial plants for processing of citrus crops to juice products created environmental problems involving disposal of peel, seeds and membranes (rags) which collectively comprise citrus waste. The major problem exists in areas with large scale processing of orange and grapefruit crops to juice, such as the State of Florida or country of Brazil. Approximately 3 to 5 million tons of citrus processing waste is produced annually in the State of Florida alone. While a small portion of the processing waste is delivered directly to cattle pastures, the majority is pressed and dried to cattle feed. The large equipment and energy costs for dewatering, drying and transportation of citrus waste are usually not recovered due to the low value of citrus waste as cattle feed.
In addition citrus waste contains significant amounts (e.g., 0.3 to 1.7%) of essential (peel) oil, composed mainly (e.g., 90-99 wt %) of d(+)-limonene. Oil vapors escape to exhaust gases during drying and have to be removed by expensive scrubbing systems or cause air pollution. Although citrus processing waste creates problems for industry, it is rich in soluble sugars and polymeric carbohydrates such as pectin, cellulose and hemicelluloses. These polymeric carbohydrates can be hydrolyzed to simple sugars by the action of acids or preferably enzymes known in the art, and these additional sugars can be fermented to ethanol, lactic acid or other products by methods known in the art. Citrus processing waste has unique aspects which differentiate it from many other plant (i.e., lignocellulosic) feedstocks. It does not contain significant amounts of lignin, instead it contains a pectin-hemicellulose matrix. The major hemicelluloses in citrus waste are arabinans and galactans, whereas in lignocellulosic materials they are xylans or glucomannans. The lack of lignin makes citrus waste easier to hydrolyze by enzymes although a complex of pectinases, cellulases and hemicellulases is needed for efficient hydrolysis. Pectin and cellulose are also highly resistant to acid hydrolysis. Another unique characteristic of citrus processing waste is its high content of soluble sugars which are held in spongy tissues as a liquid solution (juice). These sugars require only collapse or disrupture of plant tissue to be efficiently released by pressing, diffusion or similar processes.
Citrus waste also contains potent antimicrobial components in peel oil. Oil is present in specialized cells (glands) of the outer part of the peel (flavedo). In order to achieve efficient fermentation to ethanol, limonene and possibly other volatile components of peel oil have to be reduced to a level below about 3,000 parts per million (ppm) (e.g., <3000 ppm; preferably to less than about 1500 ppm (e.g., <1500 ppm)); the exact inhibitory concentrations of limonene depend on the tolerance of individual microorganisms employed in the fermentation process. It is also desirable to recover limonene and preferably peel oil because they are valuable industrial chemicals and flavoring agents.
Treatments of citrus waste prior to saccharification need to retain relatively high sugar concentrations present in this waste since higher concentrations of ethanol and other fermentation products then accumulate with attendant savings in fermentation and recovery costs. Therefore the addition of water or other diluents during the process need to be reduced to a minimum.
Since peel oil is concentrated in specialized cells, these cells need to be pierced or ruptured by mechanical or other means to free the oil from citrus waste tissue. This is currently accomplished by piercing or squashing the peel of citrus fruit before or during extraction of juice. The surface of the fruit is washed with a stream of water and the resulting emulsion is separated by decanting and preferably by centrifugation. These methods remove approximately 50% of the oil from peel, but the residual concentration (3,500-15,000 ppm) is still too high to allow fermentation of citrus waste to ethanol or other products.
One of the current methods which can be applied to removal of residual peel oil involves additional disintegration of citrus waste to fine particles and separation of the oil rich fraction by flotation or centrifugation. Other methods include solvent, detergent or other chemical extraction of citrus waste slurries. All these methods suffer from extensive dilution of waste and sugars with water, detergent solutions or organic solvents. The organic solvents and detergents are often toxic or deleterious to fermentation microorganisms and the environment.
While citrus waste at 70-85% moisture contains sufficient amounts of water to strip volatile oil components when this moisture is converted to steam by indirect heating (U.S. Pat. No. 3,966,984), the indirect heating of undiluted citrus waste is a very inefficient process. Irregular solid pieces of peel and other particles of citrus waste do not have good contact with the surfaces of indirect heat exchangers and the slimy nature of some citrus waste components leads to fouling of heat exchanger surfaces resulting in further decreases in heat transfer rate and efficiency.
Efforts to improve poor contact with heat exchanger surfaces have involved blending waste with nontoxic heat transfer liquids such as water or by decreasing the particle size by shredding, milling or other particle size reduction methods. However, dilution of citrus waste with water is not an acceptable solution because it excessively dilutes the sugars for subsequent saccharification and fermentation. The comminution of peel to small particles does not satisfactorily solve the heat transfer problem because a very viscous paste is formed by this method. This viscous paste does not mix well and does not efficiently transfer heat from the surface of the heat exchanger to the bulk of citrus waste.
The need thus exists for development of new processes for a pasteurization treatment of citrus wastes at high concentration of solids and preferably under conditions mild enough to allow removal and recovery of peel oil with minimal degradation of limonene and other components and to facilitate and allow efficient enzymatic saccharification and fermentation of citrus waste.
We have determined that a combination of indirect and direct heat exchange is more efficient in heating citrus waste particles to temperatures necessary to rupture plant tissues and release volatile oil components. The indirect heat exchanging surface increases heat input to the process without diluting the citrus waste by steam condensate, and hot gas or vapors can enter between citrus waste particles and efficiently increase their temperature (direct heat exchange).
We have also determined that one of the devices suitable for our process is a jacketed and heated screw conveyor equipped with concurrent or countercurrent steam or hot gas injection. However other heated high solids mixers/conveyors such as pug mills, helical ribbon mixers or similar devices can also be used.